Rosenbaum et al. U.S. Pat. No. 5,103,387 issued Apr. 7, 1992, entitled "High Voltage Converter", and U.S. Pat. No. 5,323,461 issued Jun. 21, 1994, entitled "Telephone Line Interface Circuit With Voltage Switching", relate to a switchmode power converter and its arrangement and functioning in a line interface circuit for a two-wire telephone subscriber line.
As described in these patents, an individual line interface circuit includes the power converter, also referred to as a controlled voltage generator, a driver circuit, and a switching arrangement between the driver circuit, the power converter, the line, and a telephone central office (C.O.) battery. A control circuit is programmed to control the switching arrangement and the power converter to provide various operating functions which may be required of the line interface circuit. These functions include, in particular, providing relatively high voltage signalling, such as ringing, on the line, and providing a controlled d.c. feed to the line.
For example, for supplying a ringing signal on the ring wire of the line, the power converter is operated from the C.O. battery to generate at its output a high voltage ringing signal waveform, determined by the control circuit, and the switching arrangement connects this output to the ring wire of the line. A ground return for the ringing signal is provided via the switching arrangement and an output of the driver circuit, which in this case is powered by the C.O. battery. Voltages at the outputs of the driver circuit are controlled or steered by the control circuit in a manner which is described and claimed in Rosch et al. U.S. patent application Ser. No. 07/868,893 filed Apr. 16, 1992, entitled "Telephone Line Interface Circuit With Voltage Control" and which is also described in Rosch et al. U.S. Pat. No. 5,274,702 issued Dec. 28, 1993, entitled "Wideband Telephone Line Interface Circuit".
For providing a controlled d.c. feed (loop current) to the line, the control circuit controls the switching circuit to couple the outputs of the driver circuit to the line, and to provide a supply voltage to the driver circuit from either the C.O. battery or, preferably, from the output of the power converter. In the latter case the control circuit controls the power converter to generate a d.c. supply voltage which is typically lower than the C.O. battery voltage, and also controls the driver circuit so that a desired d.c. feed or loop current is maintained on the line in an off-hook state of a subscriber's telephone connected to the line.
Providing the power converter as a part of each individual line interface circuit in this manner provides distinct advantages, for example in that different high voltage signalling waveforms, e.g. ringing signal waveforms and cadences, message waiting signalling, and coin signalling, can be readily provided under software control, and d.c. feed or loop currents can be tailored to the characteristics of, and to the telephone services provided on, each subscriber line. It also presents several challenges.
For example, because the power converter is provided as a part of the individual line interface circuit and hence on a line card which must be accommodated within a predetermined limited physical space, the power converter itself must be physically small and efficient. In addition, the line interface circuit must meet criteria for spectral energy transmitted on the two-wire line; in particular spectral energy transmitted on the line in a frequency band from 4 kHz to 270 kHz must be very low to meet accepted standards.
In order to meet these challenges, the power converter as described in the patents referred to above is a switchmode power converter operating at a fixed high frequency (640 kHz) with a variable pulse width or duty cycle. The use of a switchmode power converter promotes efficiency. The use of a fixed frequency enables control signals to be easily generated and synchronized from clock signals used by other parts of the line interface circuit. The use of a fixed frequency also ensures that undesired switching energy is limited to the region of this fixed frequency and its harmonics, and thus can be removed by narrow band filtering so that it does not disturb other subscriber lines (due to crosstalk) or subscriber equipment coupled to the line. Narrow band filtering has fewer side effects on service performance than wide band filtering. The use of a high frequency allows the energy storage components of the power converter to be small and relatively inexpensive, and enables operation above the critical frequency band mentioned above.
The power converter must have a relatively high power capability, because high voltage signalling functions such as ringing typically require power levels of 5 to 20 W for full compliance with performance specifications over a wide range of subscriber telephone and terminal loads. However, for most of the time the operation of the power converter involves the delivery of relatively low power levels, for example less than 0.5 W for on-hook situations (idle or on-hook transmission), 0.5 to 1.5 W for off-hook POTS (plain old telephone service) situations, and less than 2.5 W for ISDN (integrated services digital network) services. Message waiting also involves the delivery of a relatively low power level for prolonged periods, but requires high voltage operation of the power converter.
Switching losses in switchmode power converters typically increase in proportion to increase in the switching or operating frequency. The high operating frequency is desired as discussed above in order to meet the requirements for high voltage signalling such as ringing; the periods of such signalling are relatively short so that the power consumption and dissipation due to switching losses are not major concerns. However, during the low power and relatively prolonged operating states discussed above, it is desirable to reduce the switching losses in the power converter, thereby reducing power consumption and power dissipation, and increasing long-term reliability as a consequence of lower operating temperatures.
An object of this invention, therefore, is to provide an improved switchmode power converter for a telephone subscriber line interface circuit which can operate with reduced switching losses.